The present invention generally relates to a composition and process for extracting metals from liquids. More particularly, this invention relates to a class of phenoxy fluoro-alcohols, their preparation, and their use as phase modifiers and solvating agents in a solvent composition for the extraction of cesium from alkaline solutions.
Many nuclear energy complexes and treatment sites have environmental problems where cesium removal is needed. Alkaline wastes containing radioactive Cesium-137, such as those stored at the Department of Energy""s Hanford, Wash., Oak Ridge, Tenn., and Savannah River, S.C. sites are examples
Among the technologies that are currently being investigated for cesium removal from alkaline nuclear waste are solid-phase sorbent methods, such as crystalline silicotitanate, and precipitation methods, such as addition of sodium tetraphenylborate to form insoluble cesium tetraphenylborate. While generally adequate, both crystalline silicotitanate and in-tank-precipitation using sodium tetraphenylborate possess certain disadvantages that prevent them from being completely satisfactory treatment technologies. As the concentration of radioactive cesium-137 that is sorbed onto a column of crystalline silicotitanate (CST) increases, the radiation and thermal heat generated produce changes in the sorption properties of the CST material leading to desorption of cesium. Flammable hydrogen gas has also been observed to evolve from CST columns that are loaded with radioactive cesium-137. Disadvantages in using sodium tetraphenylborate for in-tank precipitation include breakdown of the inherently unstable tetraphenylborate to liberate benzene, potentially at unexpected rates and places in the treatment process.
Liquid-liquid handling methods, such as solvent-extraction, on the other hand, can be designed and used in a manner to minimize chemical and radiolytic breakdown of the reagents involved in the separation process. The reagents used in solvent extraction can be designed to have high chemical and radiolytic stability, and use of contacting equipment such as centrifugal contactors can minimize the exposure time of the solvent to high radiation fields, thus extending the useful life of the solvent. Solvent extraction processes can contribute great flexibility in being able to treat large volumes of highly radioactive waste in a safe manner, decontaminate the waste to a high level, and enable the extracted cesium to be delivered to a suitable waste form such as vitrified glass. However, there are currently no practical, developed solvent extraction processes for the removal of cesium directly from the tanks with the waste in high-salt alkaline form that the Applicants are aware of; only acid-side extraction has been addressed as described in Dietz et al. U.S. Pat. No. 5,888,398, Mar. 30, 1999, and the use of acid-side solvent-extraction technology to treat the alkaline tanks would require acidification of the wastexe2x80x94a costly option. In addition, the selectivity for cesium over sodium for the process described in U.S. Pat. No. 5,888,398 is generally less than 100, making separation of cesium from waste solutions that are highly concentrated in sodium relative to cesium inefficient. The ratios of the sodium to cesium concentrations in alkaline wastes such as those stored at the Department of Energy""s Hanford, Wash., Oak Ridge, Tenn., and Savannah River, S.C. sites are generally in excess of 10,000.
Accordingly, the present invention alleviates the necessity of adding acid or other substances to the waste since cesium extraction may be effected directly from the waste matrix. Also, the present invention is highly selective for cesium over sodium (selectivity generally greater than 10,000). As such, the present invention could play a key role in a grand treatment scheme for alkaline nuclear wastes, especially wastes with a high concentration of competing alkali metal cations.
A further problem to be solved is the need for a method which regenerates the extractant by utilizing a safe and cost-effective stripping procedure, and which avoids further generation of waste. Such methods should also release cesium from the extractant solvent without employing highly concentrated mineral acids, solvent evaporation, or distillation, or contacting of the solvent with cation exchangers. Thus, the present invention comprises a solvent extraction and stripping process cycle for the removal of cesium from alkaline tank waste. After the solvent is stripped of cesium, the solvent can be recycled in a continuous extraction and stripping process cycle.
Previously reported extractants have generally possessed insufficient selectivity or extraction power to remove cesium from a matrix concentrated in competing alkali metal cations. In addition, earlier extraction solvents involved difficulties with stability, stripping, or phase disengagement. Thus, no other candidate solvent system has emerged as a serious contender for the targeted application.
In U.S. patent application Ser. No. 09/146,800 filed Sep. 3, 1998 and hereby incorporated by reference, a solvent composition and process for extracting cesium from alkaline solutions, in particular alkaline nuclear waste solutions containing high concentrations of sodium nitrate, was described. The solvents described are composed of a calix[4]arene-crown ether extractant (most preferably calix[4]arene-bis-(tert-octylbenzo-crown-6) ether), an alkylaryl ether alcohol phase modifier, and a diluent. The calixarene-crown extractant complexes cesium cation (Cs+), and extracts it into the solvent phase as an ion-pair with, an anion such as nitrate to balance the positive charge. The alkylaryl ether alcohol phase modifiers described were a class of alkylphenoxy alcohols designed to improve the cesium extraction strength, prevent third-phase formation, and provide good phase-coalescence behavior. It is desirable to have as potent a modifier as possible, so that high cesium distribution ratios can be obtained at low extractant concentrations (as the calixarene extractants are expensive). Of the modifiers listed and tested in U.S. patent application Ser. No. 09/146,800, 1-(1,1,2,2-tetrafluoroethoxy)-3-(4-tert-octylphenoxy)-2-propanol (shown in Formula (I) below) was found to be especially effective with respect to affording a good balance of high cesium extractive strength and good phase-coalescence behavior. This modifier, a phenoxy fluoro-alcohol, code named xe2x80x9cCs-3xe2x80x9d, was recently found to be especially effective for cesium extraction from high sodium/low potassium alkaline nuclear waste such as that stored at the U.S. Department of Energy""s (DOE""s) Savannah River Site (SRS), giving rise to good (e.g.,  greater than 10) cesium distribution ratios at low (0.01 Molar) concentrations of the calixarene-crown ether extractant. The magnitude of the cesium distribution ratio was also dependent upon such factors as temperature and the volume ratio of the aqueous and organic phases. 
However, as shown in FIG. 9, the Cs-3 modifier was found to slowly degrade following prolonged contact with an SRS alkaline waste simulant (1.75 molar in free hydroxide), with a concomitant erosion in cesium extraction efficiency as measured by the cesium distribution ratio, DCs, making this modifier less than optimum for the sought solvent extraction process. Unlike the non-fluorinated modifiers described in U.S. patent application Ser. No. 09/146,800, Cs-3 contains a base-sensitive functional group (the tetrafluoroethoxy moiety, xe2x80x94OCF2CF2H). Following prolonged contact with strongly alkaline ( greater than 1 molar in free hydroxide) solutions, the Cs-3 modifier present in the solvent appears to react with itself in a condensation reaction, with concomitant loss of one tetrafluoroethoxy moiety to form a larger molecule which is ineffective as a modifier (a proposed mechanism is shown in FIG. 10). The solvent thus loses potency with respect to cesium extraction efficiency as a function of exposure time to hydroxide. The rate of modifier decomposition increases with temperature, and the rate of decomposition is also likely dependent on other factors such as the modifier concentration in the solvent, as well as the concentration of base. The other modifiers listed in U.S. patent application Ser. No. 09/146,800 that do not contain the 1,1,2,2-tetrafluoroethoxy moiety do possess good alkaline stability, but they are not as effective as the Cs-3 modifier with regard to both cesium extraction strength and phase-coalescence behavior from alkaline nitrate solutions, particularly solutions simulating SRS alkaline nuclear waste.
It is believed that the effectiveness of the Cs-3 modifier may be due to the properties of the alcohol (hydroxyl) functional group. The electron-withdrawing nature of the tetrafluoroethoxy moiety and its proximity to the alcohol (xe2x80x94OH) functionality increases both the acidity and hydrogen-bond donor ability of the hydroxyl group. In neutral and acidic aqueous media, the hydrogen-bonding capability of the hydroxyl group of phenoxy fluoro-alcohol modifier Cs-3 may enhance solvation of both the calixarene-crown ether-Cs+ complex and the associated anion such as nitrate. Also, anion solvation would enhance extraction by lowering the energy required to transport the anion (such as nitrate) from the aqueous phase into the organic (solvent) phase. However, when the solvent containing the calixarene-crown ether and the Cs-3 modifier are used to extract cesium from strongly alkaline solutions, hydroxide, which can be co-extracted into the solvent phase (though not as easily as nitrate) can abstract the weakly acidic proton (hydrogen atom) from the hydroxyl group of the Cs-3 modifier. The deprotonated modifier, as an alkoxide anion, can either be an effective counter-anion to the calixarene-crown ether-Cs+ complex, or can slowly react with excess Cs-3 modifier that is in the solvent to afford the condensation product shown in FIG. 10. The carbon atom to which the tetrafluoroethoxy group is attached is strongly electropositive and can be subject to nucleophilic attack. In the case where the alkoxide anion of Cs-3 is the nucleophile, a condensation reaction occurs in which a tetrafluoroethoxy group is eliminated to afford the condensation product. Over prolonged contact with alkaline solutions, especially at elevated temperatures, the Cs-3 modifier is thus consumed as it is converted to the condensation product shown in FIG. 10.
It should be noted that when the calixarene-crown ether extractant is not present in the solvent, the Cs-3 modifier is much more stable to alkaline solutions. This is because without the calixarene-crown ether extractant present, there is very little cesium or potassium ion extracted into the solvent phase, and accordingly very little hydroxide that would serve as the counter-anion co-extracted into the solvent phase that could react with the weakly acidic hydroxyl group of the Cs-3 modifier.
Accordingly, there is a need in the art for a phase modifier that possesses a high cesium extraction performance level similar to that of the Cs-3 modifier as previously described, but that is more chemically stable than the Cs-3 modifier to alkaline media.
In view of the above need, an object of the present invention is to provide a composition and process for the separation of cesium from alkaline waste solutions that contain large excesses of other alkali metal ions such as sodium and potassium.
A further object of the present invention is to provide a practical alkaline-side cesium extraction process.
It is also an object of the present invention to provide an improved composition and method for extracting cesium from acidic solutions.
Another object of the present invention is to provide a practical method for clean up of nuclear fission byproducts, such as that contained in waste tanks stored at DOE""s Hanford, Wash., Savannah River, S.C. and Oak Ridge, Tenn. sites.
Yet another object of the present invention is to provide a method and composition for extracting cesium from hydrometallurgical process solutions.
Another object of the present invention is to provide a composition and method, which is a substantial component of a combined process for the removal of technetium and strontium, as well as cesium, from alkaline waste.
A further object of this invention is to provide for a new class of phenoxy fluoro-alcohols which serve as phase modifiers for use in the solvent extraction of cesium from aqueous solutions and which behave synergistically with the actual cesium extractant (a calixarene-crown ether), and function to greatly increase the extraction strength for cesium and maintain good solvation by preventing precipitation and third phase formation.
It is another object of this invention to provide phenoxy fluoro-alcohol phase modifiers which possess good stability to aqueous alkaline solutions and do not degrade to a condensation product as those previously described when in prolonged contact with strongly alkaline solutions, such as alkaline nuclear waste solutions.
It is still a further object of this invention to provide for a method for making phenoxy fluoro-alcohol phase modifiers.
Briefly, the present invention is a phenoxy fluoro-alcohol of Formula (II): 
wherein for n=1 to 4 (preferably n=2 to 4), X=a hydrogen atom or a fluorine atom, R2 and R6 are each independently a hydrogen atom or C1 alkyl, R4 is a hydrogen atom or C1-C12 alkyl when R3 and R5 are each independently selected from a hydrogen atom or C1-C2 alkyl, and R3 and R5 are the same or different and each is hydrogen or C3-C12 alkyl when R4 is a hydrogen atom or C1 alkyl. Another embodiment is the phenoxy fluoro-alcohol according to Formula (II) in which n=2, X=a hydrogen atom, R2, R3, R5, and R6 are all hydrogen atoms, and R4 is a C1-C12 straight-chain or branched alkyl group, preferably selected from the group consisting of n-propyl, isopropyl, CH3CH2CH2CH2-(n-butyl), CH3CH2(CH3)CH-(sec-butyl), (CH3)2CHCH2-(iso-butyl), (CH3)3C-(tert-butyl), CH3CH2CH2CH2CH2-(n-pentyl), (CH3)2CHCH2CH2-(isoamyl), (CH3)3CCH2-(neopentyl), CH3CH2C(CH3)2-(tert-amyl), CH3CH2CH2CH2CH2CH2-(n-hexyl), CH3CH2CH2CH2CH(CH2CH3)CH2-(2-ethylhexyl), (CH3)3Cxe2x80x94CH2xe2x80x94C(CH3)2-(tert-octyl), and CH3CH2CH2CH2CH2CH2CH2CH2-(n-octyl).
The invention also provides a process for the preparation of a phenoxy fluoro-alcohol according to Formula (II) as above comprising the steps of a) reacting a phenol of Formula (III): 
wherein R2 and R6 are each independently a hydrogen atom or C1 alkyl, R4 is a hydrogen atom or C1-C12 alkyl when R3 and R5 are each independently selected from a hydrogen atom or C1-C2 alkyl, and R3 and R5 are the same or different and each is hydrogen or C3-C12 alkyl when R4 is a hydrogen atom or C1 alkyl, with a glycidyl fluoroalkyl ether of Formula (IV): 
wherein n=1 to 4 (preferably n=2 to 4), X=a hydrogen atom or a fluorine atom, and (b) separating the phenoxy fluoro-alcohol obtained from the reaction mixture. Preferably, the process for the preparation of a phenoxy fluoro-alcohol comprises the steps of a) reacting a phenol of Formula (III) as above wherein R2, R3, R5, and R6 are all hydrogen atoms, and R4 is a C1-C12 straight-chain or branched alkyl group, with glycidyl fluoroalkyl ether of Formula (IV) as above in which n=2 and X is a hydrogen atom, and b) separating the phenoxy fluoro-alcohol obtained from the reaction mixture. R4 of the phenol according to Formula (III) is preferably selected from the group consisting of n-propyl, isopropyl, CH3CH2CH2CH2-(n-butyl), CH3CH2(CH3)CH-(sec-butyl), (CH3)2CHCH2-(iso-butyl), (CH3)3C-(tert-butyl), CH3CH2CH2CH2CH2-(n-pentyl), (CH3)2CHCH2CH2-(isoamyl), (CH3)3CCH2-(neopentyl), CH3CH2CH2CH2CH(CH2CH3)CH2-(2-ethylhexyl), (CH3)3Cxe2x80x94CH2xe2x80x94C(CH3)2-(tert-octyl), and CH3CH2CH2CH2CH2CH2CH2CH2-(n-octyl).
Furthermore, the present invention is directed to a composition for extracting cesium from alkaline solutions comprising a) a phenoxy fluoro-alcohol of Formula (II) as above, b) an alkane-soluble calixarene-crown ether of Formula (V): 
c) a water-immiscible hydrocarbon diluent, and d) an aliphatic amine, preferably a tertiary amine of formula (CnH2n+1)3N in which n=6 to 12, most preferably where n=8. For the phenoxy fluoro-alcohol of Formula (II), it is preferable that n=2, X=H, R2, R3, R5, and R6=H, and R4 be an alkyl group of 4-6 carbon atoms. It is most preferable that R4 be a sec-butyl group. This phenoxy fluoro-alcohol would have the chemical name 1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol.
The invention also provides a process for extracting cesium and other metals from alkaline waste solutions comprising the steps of a) contacting a volume of an aqueous feed solution containing cesium with a solvent comprising: an alkane-soluble calix crown ether according to Formula (V) above, a water-immiscible hydrocarbon diluent, an aliphatic amine, preferably a tertiary amine of formula (CnH2n+1)3N in which n=6 to 12 (most preferably where n=8), and a phenoxy fluoro-alcohol of Formula (II) above;
b) contacting a volume of the solvent obtained above containing the portion of cesium, with a volume of a scrubbing solution, comprised of dilute mineral acid having between 30 and 200 millimolar concentration, thereby removing substantially all the other alkali metal ions (such as sodium and potassium), while retaining in the solvent a substantial portion of the cesium values; c) contacting a volume of the solvent obtained after scrubbing above with a volume of a stripping solution, comprised of pure water, dilute salt, or dilute mineral acid having up to 10 millimolar concentration, thereby removing the cesium values from the solvent phase into the stripping solution to make the organic solution containing the calixarene-crown ether, the phenoxy fluoro-alcohol, and the tertiary amine available for reuse.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and advantages may be realized and attained by means of the instrumentalities and combinations particularly pointed out herein and in the appended claims.